The invention relates to a pattern selecting mechanism for sewing machines, in which two cam groups are disposed within a limited space of an arm frame of a sewing machine, which are different in speed reduction ratios with respect to a main shaft thereof, whereby various kinds of patterns are made available in cooperation with an automatic feed arm.
In a sewing machine housing the pattern cams therein, a needle bar is given lateral swinging amplitude by one cam selected from a pattern generating cam group to be rotated at speed reduction with respect to the main shaft. There have conventionally been two thinkings in designing thoughts of said pattern cam built-in sewing machine. One of them is to make much account of applicability of the stitching pattern to make large the speed reduction ratio of the pattern cam by neglecting a consequently large volume of the pattern generating cam group. The other is to make much account of a small volume of the pattern cam group to make small the speed reduction ratio of the pattern cam by sacrificing the applicability of the stitching pattern. That is, since the pattern generating cam must not move the needle bar in a swinging direction during penetration of a needle into a fabric under sewing, the needle bar should be held at predetermined position while an upper shaft of the sewing machine rotates at least 1/2 rotation. Therefore, if 1/a is a reduction ratio of a pattern cam, the pattern cam has a configuration of at least 1/2.times.360.degree./a=180.degree./a from the rotation center thereof, and has a number of equal circumferential faces defined by "a", which are spaced from each other by 360.degree./a and are generally of different radii and are connected to each other by the same number of circumferential faces.
"a" of the reduction ratio 1/a of the cam is a stitching number to be controlled by the cam during one rotation (hereafter caled as "1 cycle stitching number"). Therefore, when the pattern is stitched with said cam, and as the reduction ratio is made large, close stitching is possible in an ordinary pattern stitching with respect to various kinds of pattern stitchings, and in the super stitching its applicability is enlarged. Herein, considering examples of the stitching patterns by the cams of the reduction ratios being 1/6 and 1/12, the stitching patterns formed with the cam having 1 cycle stitching number being 6 and the reduction ratio of 1/6, are shown in FIGS. 1 to 3. In case of a simple zigzag stitching, such as shown in FIG. 1, 3, cycles are repeated during one rotation of the cam in reference to the right and left stitchings. In case of a tricot stitching such as shown in FIG. 2, stitchings, of 1 cycle shown with the numerals 1 to 6 in the same are formed during one rotation of the cam. In case of a honey stitching being one of super stitchings such as shown in FIG. 3, stitchings shown with the numerals 1 to 6 are formed during one rotation of the cam, and by this repetition the pattern as shown in the same may be formed.
FIGS. 4 and 5 show examples of stitching patterns which are formed with a cam having 1 cycle stitching number being 12 and the reduction ratio of 1/12. In a roam stitching shown in FIG. 4, a unit pattern shown with the numerals 1 to 12 may be formed during one rotation of the cam. If a pattern similar to the roam stitching is formed with such a cam, e.g., of the reduction ratio being 1/6, it is preferable in this kind of the pattern that a distance between the stitchings is close to a certain extent. However, if the stitching is performed with a distance being close to a certain extent as mentioned, the unit pattern is shortened in the feeding direction so that the stitching pattern is not preferable in shape.
FIG. 5 shows a pattern of the super stitching by the cam of the reduction ratio being 1/12. This pattern is not specially named. The stitchings shown with the numerals 1 to 12 are formed during one rotation of the cam, and the stitching pattern as shown in the same is formed by repeating the stitching. However this stitching pattern cannot be formed with the cam of 1/6 reduction ratio. Accordingly, when making much account of the applicability of the stitching pattern, it is preferable to make large the reduction ratio of the pattern cam, but when taking the sizing precision on the production into consideration, the actual size of a lift of the cam giving the maximum lateral amplitude to the needle bar cannot be made less than a certain predetermined ratio with respect to the maximum lateral amplitude of the needle bar, and accordingly the maximum outer diameter size of the cam cannot be made less than a certain size either, due to relation with the pressure angle, and the minimum size is settled down to a certain size. If only the reduction ratio is changed with respect to the main shaft of the sewing machine while maintaining other conditions constant, the maximum outer diameter of said cam is required to be larger as the reduction ratio is made larger in relation with the pressure angle of the cam face. When the cams are superimposed within the machine frame, volume occupying the narrow space within the machine frame is large so that the space for other built-in mechanisms is disadvantageously limited.
Thus, there have been conventionally the above mentioned two thinkings in the designing thoughts of the pattern generating cams. One of them makes large the reduction ratio of the pattern cam in view of the applicability of the stitching pattern, ignoring increase of the volume of the pattern generating cam group. The other makes small the reduction ratio of the pattern cam in view of the small volume of the pattern generating cam group, ignoring the applicability of the stitching pattern. These thoughts have still merits and demerits.